Pinnularia australoborealis Van de Vijver & Zidarova 2011, sp. nov.

Pinnularia australoborealis Van de Vijver & Zidarova, sp. nov. (Figs 18–25)

Valvae stricte lanceolatae marginibus convexibus, apicibusque truncatis, subrostratis. Dimensiones (n=15): longitudo 51–61 µm, latitudo 12.5–14.0 µm, ratio longitudinis/latitudinis 3.9–4.3. Area axialis moderate angusta, linearislanceolata, 1/6 latitudinis valvae, leviter dilatans ad aream centralem. Area centralis paene rotundata ad rectangularis. Fascia abest. Raphe moderate lateralis. Rami externi raphis leviter ad distincte curvati. Terminationes proximales leviter unilateraliter deflexae, in summa reflexae ad paene rectae. Fissurae distales unciformes. Striae transapicales latae, leviter ad moderate radiatae in media parte valvae, ad parallelae in apices, 4–5 in 10 µm.

Valves moderately large, strictly lanceolate with convex margins and subrostrate, truncated apices. Valve dimensions (n=15): length 51–61 µm, width 12.5–14.0 µm, length/width ratio 3.9–4.3. Axial area moderately narrow, linear-lanceolate, 1/6 of the valve width, slightly widening towards central area. Central area almost rounded (see, for instance, Fig. 20) to mostly rectangular, lacking fascia, since asymmetrically bordered by 1– 3 shortened striae on each side. Raphe moderately lateral, with outer raphe branches weakly to distinctly curved. Proximal raphe endings slightly unilaterally deflected but at end reflected to almost straight (Fig. 25), terminating pore-like. Distal fissures typically sickle-shaped. Transapical striae broad, weakly to moderately radiate in middle to parallel near apices, 4–5 in 10 µm.

Type:— Byers Peninsula, Livingston Island, South Shetland Islands, sample LIV16 A, leg. A. Quesada, coll. date 16/12/2006, slide no. BR-4233 (holotype BR), slide PLP-183 (isotype University of Antwerp, Belgium), BRM-7/96 (isotype BRM).

Habitat and Distribution: —On Byers Peninsula, Pinnularia australoborealis was only observed in some samples taken in flooded algal mats, living between stones. It was also observed on James Ross Island, situated in the northern Weddell Sea, on the eastern side of the Antarctic Peninsula (Van de Vijver & Kopalová, unpubl. results).

Etymology:—Refers to the combination of the southern distribution (‘ australo ’ from the Latin australis meaning southern) and its resemblance to P. borealis.

Observations:—Based on its valve dimensions, valve outline and raphe structure, P. australoborealis shows an intermediate position between P. rabenhorstii and P. borealis (Tables 1 & 2) but is too large to be included within the P. borealis group (length P. borealis s.l.: 24–42 µm, width: 8.5–10 µm). The combination of truncated valve apices, the typically convex valve margins and the larger valve width make conspecificity with P. borealis var. islandica Krammer (2000: 25, 209) unlikely since the latter has more parallel valve margins, broadly rounded apices and a valve width of 10–12 µ m (as compared to 12.5–14.0 for P. australoborealis). The raphe structure with the proximal raphe endings separates P. australoborealis from the P. rabenhorstii complex. Pinnularia rabenhorstii var. franconica Krammer (2000: 22, 209) can be distinguished based on the valve outline with parallel margins compared to the convex margins in P. australoborealis and only simply deflected proximal raphe endings (Table 2). Pinnularia subrabenhorstii Krammer (2000: 23) has parallel, undulated valve margins, a feature never observed in P. australoborealis. Other taxa such as P. rabenhorstii var. subantarctica and P. australorabenhrostii have different valve outlines and/or different valve dimensions (Table 2). Pinnularia rabenhorstii var. subantarctica has almost parallel valve margins and rounded ends in comparison to the clearly convex margins and subrostrate apices of P. australoborealis. Pinnularia australorabenhorstii is larger in width (16–17.5 µm vs. 12.5–14.0 µm in P. australoborealis) with more densely spaced striae (6–8 vs. 4–5 in 10 µm in P. australoborealis).